1. Field of the Invention
The present invention relates to an improved image reader for reading bound documents such as books, magazines and the like.
2. Description of the Related Art
Image reading devices have been commercialized as so-called book scanners. In this device, an image sensing unit is disposed above a document platen, and an open space comprising a height of several tens of centimeters is maintained between the document platen and the image sensing unit. When reading a bound document such as a book or magazine (hereinafter referred to as "book document"), a user sets a book document on the document platen in an open-faced state facing upward. When the start key is turned ON, scanning of the document surface (surface to be read) begins, and the read image is sequentially output to an external device in parallel with the scanning. If the external device is a printer, a reproduced image is formed in real time.
When a book scanner is used, the operation of reading a book document comprising a plurality of pages is accomplished with improved efficiency because the pages can be turned on the document platen. Furthermore, damage to the book is markedly reduced compared to when a book is placed in an open faced state facing downward. The aforesaid book documents include filed documents.
Unlike sheet documents, book documents often have a curved document surface. When the document surface is curved, the read image is distorted in accordance with the state of curvature. When outputting a read image to an external device, it is necessary that the read image is corrected for distortion, i.e., to output a read image having an image quality similar to a sheet image.
The slit optical projection method (hereinafter referred to as "optical section method") used for three-dimensional measuring is one measuring method used to measure the state of curvature of a document surface. This method exposes an object surface to slit light (a light beam having a linear cross section), projects the object surface from a direction different from the exposure direction of the slit light, and checks the shape of the exposed slit light, and is capable of high precision measurement. If the object surface is flat, the exposure portion is linear, whereas the exposure portion is a curve when the exposure surface is curved. Similar measurement can be accomplished by scanning the document surface with a spot light instead of slit light.
In conventional book scanning, a special measurement light source is provided, and curved conditions are measured by slit light of the visible band. Correction of a read image is accomplished based on these measurement results (U.S. patent application Ser. No. 5,084,611).
When reading a document image, the document surface must be illuminated by visible light. Conventionally, reading of a document image and measurement of the state of curvature cannot be accomplished simultaneously because visible light is used to measure the state of curvature as mentioned above. These two operations cannot be accomplished simultaneously because the slit light is nullified by the aforesaid illumination. Therefore, reading of a document image is accomplished after the state of curvature has been measured, which is disadvantageous inasmuch as a long time is required to read a single page. A further disadvantage is that suitable distortion correction cannot be achieved when the state of curvature changes after curvature measurement has been accomplished.
In addition, since the state of curvature of both pages of an open faced book document is measured, a special measurement light source is provided separately as an illumination light source, thereby increasing the size of the book scanner.